Overheating or thick white smoke? Switch off now
Pull over somewhere safe and turn the engine off. Leave the bonnet shut while it's hot, and never open the coolant cap on a hot engine: a failing gasket can push combustion gas into the cooling system, and pressure builds fast. If you've stopped somewhere dangerous, hazards on, and call 111 if anyone is hurt or at risk.
What does a head gasket do?
It's a thin seal clamped between the bottom half of the engine (the block) and the top half (the cylinder head). It keeps the burning fuel and air inside each cylinder, and keeps apart the coolant and oil passages that run through both halves.
When it fails, those things cross over. Coolant leaks into a cylinder and leaves as white smoke, combustion gas pushes into the coolant, or oil and coolant mix. Sometimes the leak is to the outside, and you see oil or coolant weeping down the side of the engine.
Head gasket signs, and what else causes them
| Could also be | How a mechanic tells | |
|---|---|---|
| White smoke that doesn't clear when warm | Condensation on a cold morning | Vapour clears as the engine warms; coolant smoke doesn't |
| Milky film under the oil cap | Condensation from short trips in cold, damp weather | A coolant-in-oil test, not the cap alone |
| Coolant disappearing, no puddle | A small external leak drying off hot parts | A pressure test of the cooling system |
| Overheating | A leak, the thermostat, the fan or the pump | A test for combustion gas in the coolant |
| Rough running, lost power | A misfire from plugs or coils | Compression and leak-down tests |
Why do head gaskets fail?
Heat, mostly. AA New Zealand names overheating as the most common cause, often after coolant loss, a blocked radiator or a thermostat that's failed. Wikipedia's head gasket entry describes how it feeds itself: gas leaking into the cooling system can trap the thermostat shut with air pockets, so the engine runs hotter and the leak gets worse.
That's why a cooling problem caught early is the smaller job, and a car that's overheating is worth stopping for every time.
What to do now
Don't drive it hot
If the gauge climbs or the smoke is heavy, have it towed or seen where it is.
Note what you've seen
Smoke colour, when it happens, how fast the coolant drops, and whether the oil looks milky.
Check levels cold
With the engine cold, look at the coolant overflow bottle and the oil dipstick, and take a photo of each.
Ask for tests before a teardown
Confirming the gasket before the head comes off saves paying for a strip-down on a hunch.
How will the mechanic confirm it?
There are a few tests. A combustion gas test (sometimes called a block test) checks the coolant for exhaust gases. A compression test shows whether a cylinder is losing pressure, and a leak-down test pumps air into one cylinder and shows where it escapes. Wikipedia notes that a failing gasket can show up as bubbles in the coolant during a leak-down test.
The gasket itself is a small part. The work is in taking the cylinder head off, checking it's flat and not cracked, and putting it all back. So ask what the quote includes: checking or machining the head, head bolts if the maker says to replace them, the thermostat and fresh coolant.
Repair it, or move the car on?
On an older car this is a real decision. Weigh the repair against the car's value, its kilometres and how the rest of it is going. Ask the mechanic for a straight view, and if you're unsure, Consumer Protection suggests getting a second opinion from another garage when you're worried a part doesn't really need replacing.
What you can do, and what to leave
You can
- Check coolant and oil levels on a cold engine and photograph them.
- Watch the temperature gauge and stop at the first climb.
- Note the smoke colour and when it appears.
Leave to the mechanic
- Pressure, combustion gas, compression and leak-down tests.
- Taking the cylinder head off and checking it.
- Refilling the cooling system and bleeding the air out.
Our take
We'd stop driving it, have the tests done before any teardown, and ask for a written quote that lists the head check, the gasket, the bolts and the cooling parts. Then we'd weigh that number against what the car is worth before saying yes.